Stars engage in vampirism to look young, hot

7:43 December 23, 2009

Scientists studying stars in the globular cluster of Messier 30, a swarm of hundreds of thousands of stars that formed 13 billion years ago, discovered that a small fraction of them appeared to be significantly younger. Astronomers dubbed the perky stars that seem to defy ageing as blue stragglers. Further probing using the Hubble Space Telescope found evidence that there were two populations of these blue stragglers.

The study will be published in the Dec. 24 issue of Nature.

“In short, we seem to have found that there are two fountains of youth for stars”, says Alison Sills, assistant professor in the Department of Physics & Astronomy at McMaster University. Blue stragglers appear to regress from “old age” back to a hotter and brighter “youth”, gaining a new lease on life in the process.

Blue stragglers have been known since the early 1950s, but how they formed remains an astrophysical puzzle. “It’s like seeing a few kids in a group photo of residents of a retirement home, and ask, ‘How did they get there?’” says Sills.

Previously, it was thought that that the less massive star in a binary system acts as a “vampire”, siphoning fresh hydrogen from its more massive companion star that allows the smaller star to heat up, growing bluer and hotter. However, the new study shows that some of the blue stragglers have instead been rejuvenated by a sort of “cosmic facelift”, courtesy of cosmic collisions. These stellar encounters are nearly head-on collisions in which the stars actually merge, mixing their nuclear fuel and re-stoking the fires of nuclear fusion. Merged stars and binary systems would both be about twice the typical mass of individual stars in the cluster.

“The observations, which agree with our models, demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster,” says Sills, who was part of an international steam that made the findings.

Using data from the now-retired Wide Field Planetary Camera 2 (WFPC2) aboard Hubble, astronomers found that these “straggling” stars are much more concentrated towards the centre of the cluster than the average star.

The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major “core collapse” that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favoured the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers.

The study was funded by grants from the Natural Sciences and Engineering Research Council, the Italian Space Agency, the Italian National Institute for Astrophysics, Space Telescope Science Institute, and the European Space Agency.

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